Hmm we have been to Venice a couple of times and other than the culture, bars and stuff I am struggling to think what you can do with kids tbh. day trips out might be your best bet. venice is very expensive for food and drink. sorry. not terribly helpful but someone had to say it. have a look at tripadvisor and see what activities they list there. good luck.
We went to the Venice area this summer with our older (teens) children.
I would agree with mummymeister - you will need to have some day trips out of the city.
We actually stayed at Lido di Jesselo to the east (not to be confused with Lido - the island just across the water where the film festival is) which is basically just a very long beach resort. They have the usual resort type attractions - a small zoo, amusements etc. Most important from our point of view was a wonderful waterpark - Aqualandia. It can get very busy on hot days (we chose a not-so-hot day and it was lovely and empty) but has things to keep all age groups amused.
We went last year when ds's were 2 and 3 years old. Had sling for both (boba 4g), so could back carry them if needed as lots of walking.
Ended up only carrying back from restaurants at night as they walked miles in the day. Took them back to hotel and all napped late afternoon 5-7pm so could head out late for dinner.
The liked the church's, boat trips, lots of ice cream, joined a painting class along the water, visited surrounding islands. We didn't go anywhere particularly for children but they are used to that. We also went long weekend, I think a week in just Venice is too long